Not only is it arbitrary, but it also changes over time. For example, when I was growing up, 28mm lenses were far more common than 24mm lenses. But these days, I doubt many people are buying 28mm lenses - all the interest is in 24mm lenses or wider.

The same could be said for 85mm. While appreciating that many people buy 85mm lenses, I suspect that the "standard" longer prime would be in the 90-135mm range (especially when you consider all of the macro lenses in that range that are sold).

I agree with the 24mm vs 28But 85mm is still by far the most popular short telephoto. B&H has almost 1400 reviews of the 85 1.8, and only 90 reviews of the 100 f/2, even though these two lenses are about the same price, same size, and deliver the same IQ

IMHO, if there was EF 75mm f/1.4 USM for the same price, then it would beat the 85/1.8

I'm getting an M. Mostly this will be my Parks (WDW) travel setup where the 5D3 is too bulky/heavy.

I'm thinking for Animal Kingdom in bringing the M with the Tamron 70-200 + 1.4 TC. I get 448mm @ f/4. Is that right?

The "M" is APS C, so you will see a angle of view 1.6 X that of a 200mm lens, or 320mm. Have you tried one with the Tamron? I would not expect it to work well with a Tamron lens, it may be extremely slow to focus. Most third party lenses do not play well with Canon's live view. There is no phase detect with a EOS M, so try first or make sure you can return it.With a huge lens, the body size makes only a slight difference, and balance may be poor. You might be far better off with a SL1 and the ability to use phase detect AF if you want a small body. Tamron Af will work reasonably well with phase detect.

Good Luck.

+1SX50'HS is a nice, small, almost pocketable, cheap super telephoto, but if you want better IQ, then you should avoid TC's on zoom lenses as well. Otherwise, you won't get a much better result than a super-zoom P&S camera.

Actually, it would (kind of) make sense if every next FL had around 2x smaller FoV, like:12mm<17mm<24mm<35mm<50mm<70mm<100mm<135mm<200mm<300mm<400mm...or10mm<14mm<20mm<28mm<40mm<60mm<85mm<120mm<180mm<250mm<350mm<500mm...

If you compare fast Canon L primes24/14~1.71 >> 1.71*1.71~2.92x35/24~1.46 >> 1.46*1.46~2.13x50/35~1.43 >> 1.43*1.43~2.04x85/50=1.7 >> 1.7*1.7=2.89x135/85~1.59 >> 1.59*1.59~2.53x200/135~1.48 >> 1.48*1.48~2.19xthen you'll see that they are not evenly spaced. The FoV difference varies from around 2x to almost 3x.

Sigma 24-70 f/2 OS HSM - great! It could be THE lens for videographers. Now let's think about it realistically:very BIG, very HEAVY, not weather-sealed (most likely);much more expensive than 24-70/2.8L'II, could be $3k+ (just look at the 120-300/2.8 ).

I'm using Sigma 150/2.8HSM since 2009. It is my favorite lens. Can't say anything bad about it. For the price/size/weight in 150-180mm range f/2.8 Macro it has no competition. The new one may be a little bit better +stabilized, but it is bigger, heavier and more expensive as well. It is an excellent outdoor portrait and tele lens too. The bokeh is very nice.

"Whether or not regions outside the focal plane appear sharp...that's DoF" - That applies to your eyes, not the original image.

Yes, it still applies to the original image. Tell me...how do you calculate your (incorrect) concept of the "DoF" of 'the original image'. I'd like to see the math behind that, if you could share it. Also, what do you even call THAT THING - because it's not the DoF, by definition.

Regardless, while a tree falling in the forest with no one around to hear it does, in fact, make a sound, a photograph without someone to look at it is a completely meaningless collection of 0's and 1's or an equally meaningless collection of developed silver halide grains in an emulsion. The moment someone views it, all of my points about CoC apply...and that, for all purposes relevant to photographs, is the real end of the discussion.

Imagine that you are a cyborg and you see the world through cameras instead of eyes. One camera has 8mp APSC sensor from 20D and another one has 21mp FF sensor from 5D2 (yes, it's weird, you must be made in China, or something). Both with 40mm f/2.8 lenses. You see everything in clearest details up to a single pixel, all of them, all the time. FF camera has 60% wider FoV, but with both lenses focused at the same distance you would see that they both produce the same DoF.This is a very simplified concept (no need to tell me that), because I don't want to waste any more time on this, but it is real and correct. That's how your camera sees it and renders the DoF. The question is not "How do my eyes deal with DoF?". It is "How the camera does it?". It may not be useful for thumbnails and snapshots, but there is a need for it in photography with extremely shallow DoF and a lot of cropping, like macro (where you can't bring it back if it's oof).